Known multiple access schemes, which allow a plurality of users to perform communication simultaneously by sharing a single radio transmission path, include code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA). An example of the TDMA technology is disclosed in patent document 1. Wideband CDMA (W-CDMA) is one known multiple access scheme for third-generation mobile phones.
In CDMA communication systems in which a plurality of channels share the same frequency band, such as the W-CDMA cellular system, individual channels used in the cell formed by a radio base station apparatus are identified by channelization codes, which are code sequences with good code-to-code identification characteristics, that is, with good orthogonality.
Although the channelization codes have good identification characteristics, the number of the codes is limited. To increase the radio communication rate, the length of the code sequence should be shortened. A shortened channelization code sequence, however, means that the code space reserved for each single code would be significantly wasted. Accordingly, the number of channelization codes to be actually used has been a subject to be reduced.
To solve the problem, the Third Generation Partnership Project (3GPP) added a fractional dedicated physical channel (F-DPCH) to the W-CDMA cellular system (non-patent document 1). In F-DPCH, each single radio slot is time-divided into ten short slots. Each single short slot is assigned to one of multiple users assigned a single channelization code, so that up to ten users can share the single code. Unlike the conventional systems, F-DPCH prevents one code from being occupied by a single user. Consequently, the channelization codes can be used efficiently.    Patent document 1: Japanese Unexamined Patent Application Publication No. 2-260929    Non-patent document 1: “3GPP TS 25.211 V 6.9.0”, [online] November 2007, 3rd Generation Partnership Project, [retrieved on Jan. 23, 2008].